1. Field of the Invention
The present invention relates to surface acoustic wave devices, such as surface acoustic wave resonators and surface acoustic wave filters, and manufacturing methods therefor, and more particularly, to a surface acoustic wave device using a Shear Horizontal type (“SH-type”) surface acoustic wave and having a structure for reducing a transversal mode spurious ripple, and a manufacturing method therefor.
2. Description of the Related Art
In surface acoustic wave devices, aluminum or alloys including aluminum as a main component have conventionally been widely used as the electrode material of an interdigital transducer (IDT). At least one IDT is disposed on a piezoelectric substrate and reflectors or reflective end surfaces are disposed at both sides of the area where the IDT is located so as to define a resonator or a longitudinally coupled resonator filter.
In such a surface acoustic wave device, it may be possible that the IDT functions as a waveguide to generate a transversal mode wave, and ripples caused by the transversal mode wave are generated in a pass band. To reduce the ripples caused by the transversal mode wave, various methods have been attempted. Those methods include a method for reducing the intersection width of IDTs and a weighting method.
A surface acoustic wave device has also been proposed in Japanese Unexamined Patent Application Publication No. Hei-11-298290, in which a quartz substrate is used, an IDT made from a metal or an alloy having tantalum (Ta), which has a larger mass than aluminum (Al), as a main component is disposed on the quartz substrate, and an SH-type surface acoustic wave is used. Since the IDT is made from a metal or an alloy having tantalum, which has a large mass, as a main component, the number of the pairs of the electrode fingers of the IDT is as small as 10 to 20, and thereby the surface acoustic wave device is made compact.
When an electrode material having a large mass-load effect, such as a material having Ta as a main component, is used, the sonic speed obtained at the area where an IDT is located becomes much lower than the sonic speed obtained around the area. Therefore, a waveguide effect is very large at the IDT portion.
Consequently, when a longitudinally coupled resonator filter is produced, ripples caused by a transversal mode wave become complicated and very large, as indicated by arrows X in FIG. 13.
As described above, as methods for removing ripples caused by a transversal mode wave from the pass band of a filter or from the vicinity of a resonant point of a resonator, a method A in which an intersection width is made small and the frequency distance between a basic-mode wave and a transversal mode wave is made large, and a method B in which the intersection width of an IDT is weighted with a cos2 function to eliminate the transversal mode wave have been conventionally attempted.
In the method A, it is necessary to set the intersection width to 10λ or less, where λ is the wavelength of a surface acoustic wave. When a quartz substrate and an IDT having 10 to 20 pairs of electrode fingers are used to provide a surface acoustic wave device, the input and output impedance exceeds 2 kΩ and is very high, so that the surface acoustic wave device cannot be used for actual products. Therefore, it is necessary to increase the number of the pairs of electrode fingers to reduce the impedance.
More specifically, whereas the surface acoustic wave device disclosed in the above-described publication uses tantalum, which has a large mass, as a main component to form electrodes and allows the number of pairs in IDTs to be reduced, when the method for reducing the intersection width is used, the number of the pairs of electrode fingers needs to be increased to reduce the input and output impedance. Therefore, the surface acoustic wave device cannot be made compact.
In the method B, weighting itself increases a loss of the surface acoustic wave device. In addition, since weighting reduces the area of an intersection-width portion, the impedance of the surface acoustic wave device becomes very high in the same way as in the method A. Therefore, to reduce the impedance, the intersection width needs to be twice as large as the required length. As a result, the surface acoustic wave device cannot be made compact.
In other words, when either the method A or the method B is used, if ripples caused by a transversal mode wave are to be reduced, the advantage of the surface acoustic wave device in reducing the size of the device as disclosed in the above-described publication is prevented from being achieved.